Amplified telecommunication system for wavelength-division multiplexing transmissions

ABSTRACT

The present invention relates to an optical amplifier and a telecommunication system including a plurality of the optical amplifiers connected in cascade, particularly adapted for a wavelength division multiplexing transmission, in which a combination of dopants in the active fiber core of the optical amplifiers enables a high signal/noise ratio to be achieved for all channels in a predetermined wavelength band, even in the presence of several simultaneously fed signals.

This is a continuation of application Ser. No. 08/367,719, filed Jan. 3,1995 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a telecommunication system includingoptical amplifiers, particularly adapted for a wavelength-divisionmultiplexing transmission (hereinafter "WDM transmission").

2. Discussion of the Related Art

For WDM transmission several channels or several transmission signalsindependent of each other are required to be sent over the same line,for example an optical fiber, by multiplexing in the optical frequencyregion. The transmitted channels can be both digital and analog and aredistinguished from each other because each is associated with a specificfrequency.

In this kind of transmission, the different channels must besubstantially equivalent to each other, that is none of them must bemore or less privileged relative to the others, in terms of signal levelor quality.

Amplifiers, in particular optical amplifiers, are required tosubstantially have the same response for all transmitted channels; inaddition, in order to transmit a high number of channels, the band inwhich the amplifier can operate must be wide.

Optical amplifiers are based on the properties of a fluorescent dopant,and in particular erbium, introduced as the dopant into an optical fibercore. When erbium is excited by administration of optical pumpingenergy, it has a high emission in the wavelength range corresponding tothe minimum light attenuation in silica-based optical fibers. When anerbium-doped fiber, where erbium is held to an excited state, is passedthrough by an optical signal having a wavelength corresponding to such ahigh emission, the signal causes the transition of the excited erbiumatoms to a lower level and an optical emission is stimulated to thewavelength of the signal itself, thereby producing signal amplification.

Starting from the excited state, the decay of the erbium atoms takesplace spontaneously and this generates a random emission constituting"background noise" overlapping the stimulated emission corresponding tothe amplified signal. The optical emission generated by admittingoptical pumping energy to the "doped" or active fiber can take place atseveral wavelengths characteristic of the doping substance to cause afluorescence spectrum in the fiber.

To achieve the greatest amplification signal by a fiber of the abovetype and a high signal/noise ratio suitable for correct reception of thesignal itself, an optical telecommunications signal is usually usedwhich is generated by a laser emitter at a wavelength corresponding tothe maximum, in the intended band, of the fluorescence spectrum curve inthe fiber incorporating the employed doping substance, or emission peak.

Erbium-doped fibers, on the other hand, have an emission spectrum with apeak of limited width the features of which vary depending on the glasssystem into which erbium is introduced as the dopant. The spectrum areaof such a high intensity is in a wavelength range contiguous to theabove peak, within the wavelength range of interest, makes the use ofoptical amplifiers for amplifying signals in a wide band possible. Knownerbium-doped fibers, however, exhibit an uneven emission spectrum, whichreduces the possibility of achieving a uniform amplification over thewhole selected band.

In order to achieve a substantially "flat" gain curve, that is asconstant a gain as is possible at the different wavelengths, noisesources due to spontaneous emission, are eliminated by filteringelements such as those described in patents EP 426,222, EP 441,211 andEP 417,441 in the name of the same Applicant.

In such patents, however, the amplifiers' behavior in the presence ofwavelength division multiplexing is not described and in addition thebehavior in the presence of several amplifiers connected to each otherin cascade has not been taken into account. The emission spectrumprofile greatly depends on the dopants present in the fiber core inorder to increase the refraction index thereof, as shown for example inU.S. Pat. No. 5,282,079, in which the fluorescence spectrum of analumina/erbium-doped fiber is shown to have a less defined peak than agermanium-erbium-doped fiber and to be shifted to lower wavelengths (themaximum is at about 1532 nm); such a fiber had a numerical aperture (NA)of 0.15.

In ECOC '93, ThC 12.1, pages 1-4, a fiber for an optical amplifier dopedwith Al and La and having a very low responsiveness to hydrogen isdisclosed. The described Al-doped fiber has a numerical aperture of 0.16and the Al-La-doped fiber has a numerical aperture of 0.30. ECOC '93, Tu4, pages 181-184 describes optical amplifiers having erbium-doped fibersand experiments that were carried out with these fibers. The cores weredoped with aluminium, aluminium/germanium and lanthanum/aluminium andthe best results appear to have been reached with Al/La-co-doped fibers.

Electronics Letters, 6 Jun. 1991, vol. 27, No. 12, pages 1065-1067,points out that in optical amplifiers having an erbium-doped fiber,co-doping with alumina enables a larger and flatter gain profile to bereached. Also described in the article are amplifiers having analumina-doped, germanium-doped and erbium-doped fiber as compared withamplifiers having a lanthanum-doped, germanium-doped and erbium-dopedfiber and it is stated that the greatest gain flattening is obtained bythe former.

In ECOC '91, TuPS1-3, pages 285-288 a fiber of the Al₂ O₃ --SiO₂ typedoped with Er and La is described for the purpose of obtaining a higherrefractive index and reducing the formation of clusters containingerbium ions. The fluorescence and absorption spectra of the Er/La-dopedfiber have proved to be very similar to those of an erbium-doped Al₂ O₃--SiO₂ fiber. A numerical aperture (NA) of 0.31 has been achieved withan erbium concentration of 23.10¹⁸ cm⁻³.

In ECOC '89, Post-Deadline Papers, PDA-8, pages 33-36, 10-14 Sep. 1989,describes experiments made with twelve optical amplifiers connected incascade using an erbium-doped fiber. A single signal wavelength of 1.536μm has been used, and it is pointed out that signal wavelength controlin the order of 0.01 nm is required for stable operation, in view of thefact that BER (Bit Error Rate) characteristics rapidly decay on changingthe signal wavelength.

U.S. Pat. No. 5,117,303 discloses an optical transmission systemcomprising locked optical amplifiers that, based on the statedcalculations, when operating in a saturated manner give a highsignal/noise ratio. The described amplifiers include an erbium-dopedfiber having an Al₂ O₃ --SiO₂ core and the use of filters is provided.The calculated performance is achieved at a single wavelength and asignal supply in a wide wavelength band offering the same performance isnot provided.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is an active fiber for an opticalamplifier capable of wavelength-division multiplexing transmission.

Another object of the present invention is an active-fiber opticalamplifier capable of efficiently amplifying signals of differentfrequencies within a band used for optical transmission.

A further object of the present invention is an optical transmissionsystem capable of simultaneously transmitting signals of differentwavelengths without significant signal loss.

The present invention contemplates an active optical fiber, for use in alaser-pumped optical telecommunications amplifier comprising a core, atleast one main fluorescent dopant in the core, and at least onesecondary dopant in the core, the main dopant and the secondary dopanthaving functional relation with each other such that the emission curveof the fiber in a predetermined wavelength band includes a plurality ofemission zones, and in the presence of optical pumping energy suppliedto the fiber, the emission curve is clear of depressions of a valuehigher than 1 dB relative to the emission value in at least one of theadjacent zones in the band.

The present invention also contemplates an active-fiber opticalamplifier, comprising at least one length of silica-based active fiber,a pump for the active fiber to supply optical pumping power at a pumpingwavelength to the active fiber, a coupler within the active fiber tocouple the optical pumping power and at least one transmission signal,at a transmission wavelength included in a predetermined transmissionband to the active fiber, the active fiber having a core doped with atleast one main fluorescent dopant and at least one secondary dopant, themain fluorescent dopant and the secondary dopant being in functionalrelation with each other such that the maximum gain variation betweentwo signals at different transmission wavelengths in the band measuredat an input power ≦-20 dBm, is lower than 2.5 dB without filtering meansassociated with the active fiber.

The present invention further contemplates a telecommunications systemfor transmitting optical signals in a predetermined wavelength band,from a transmission station to a receiving station, comprising anoptical fiber adapted to connect the transmission station and receivingstation, and at least two active-fiber optical amplifiers connected inseries along the optical fiber, at least one of the optical amplifierscomprising a silica-based active optical fiber having a core doped withat least one main fluorescent dopant and at least one secondary dopantin appropriate amounts such that the receiving station receives signalswith at optical signal/noise ratio, measured at a 0.5 nm filter width,not lower than 15 dB for signals of a wavelength included in the band.

BRIEF DESCRIPTION OF THE DRAWINGS

The manner by which the above objects, features, and advantages of thepresent invention are attained will be explained in detail in thefollowing detailed description when considered in view of theaccompanying drawings, wherein:

FIG. 1 shows a diagram of an optical amplifier;

FIG. 2 is a diagram of an optical amplifier having a notch filter;

FIG. 3 is a diagram of an experimental configuration for determinationof the spectral emission graphs for different types of optical fiber;

FIG. 4 shows the spectral emissions for different types of active fibersas determined by adopting the experimental configuration in FIG. 3;

FIG. 5 shows the gain curves of an amplifier of FIG. 1, for signals atdifferent wavelengths and two different levels of input power, when thefiber of the amplifier is made in accordance with the present invention;

FIG. 6 shows the gain curves of an amplifier of FIG. 2, for signals atdifferent wavelengths and three different levels of input power, whenthe fiber of the amplifier is made in accordance with the presentinvention;

FIG. 7 shows the gain curves of an amplifier seen in FIG. 2, for signalsat different wavelengths and three different levels of input power, whenthe amplifier uses a conventional fiber;

FIG. 8 shows an experimental transmission system comprising severalamplifiers in cascade, in the presence of two signals at differentwavelengths multiplexed in the same line;

FIG. 9 shows BER (Bit Error Rate) graphs detected by using theexperimental system of FIG. 8, with different amplifiers;

FIG. 10 shows an experimental transmission system comprising severalamplifiers in cascade, in the presence of four signals at differentwavelengths multiplexed in the same line;

FIG. 11 shows the signal power levels at the input of the first lineamplifier in the experimental system of FIG. 10, using amplifiersaccording to the present invention;

FIG. 12 shows the signal power levels at the input of the second lineamplifier in the experimental system of FIG. 10;

FIG. 13 shows the signal power levels at the input of the third lineamplifier in the experimental system of FIG. 10;

FIG. 14 shows the signal power levels at the input of the fourth lineamplifier in the experimental system of FIG. 10;

FIG. 15 shows the signal power levels at the preamplifier input in theexperimental system of FIG. 10;

FIG. 16 shows the signal power-levels at the preamplifier input in theexperimental system of FIG. 10, using amplifiers of a known type.

DETAILED DESCRIPTION

According to the present invention, it has been found that a particularcombination of dopants in the core of an active optical fiber makes itpossible to produce a fiber having a high numerical aperture togetherwith an emission spectrum the features of which enable opticalamplifiers to be made which, particularly in a wavelength multiplexingsystem, give a uniform response to the different wavelengths in theprovided wavelength range. This is true for the case of a singleamplifier and also the case of several amplifiers connected in cascade.

The present invention, in one aspect, relates to a method of achievingthe control of the optical signal/noise ratio on reception, in apredetermined wavelength band, in an optical telecommunication system.The system includes an optical transmitter, an optical receiver, anoptical fiber line connecting the transmitter and receiver, and at leastone active-fiber optical amplifier interposed along the line. The activefiber exhibits an emission curve having a high-emission zone in awavelength range including the predetermined wavelength band, insidewhich an emission depression relative to the adjacent zones is present.An improvement includes eliminating or reducing the depression in theemission curve through selection and proper dosing of the dopants in theactive fiber.

In particular, the predetermined wavelength band is between 1530 and1560 nm and preferably between 1525 and 1560 nm. Preferentially, theoptical signal/noise ratio, measured at a 0.5 nm filter width, isgreater than 15 dB. In a preferred embodiment, the system is comprisedof at least two active-fiber optical amplifiers interposed in seriesalong the optical fiber line.

In a preferred embodiment of the method of the invention, the dopantselection in the fiber comprises the use of a main fluorescent dopantand at least one secondary dopant interacting with the main dopant inthe glass matrix of the active fiber, for reducing the depression to avalue lower than 1 dB relative to the emission value in at least one ofthe adjacent zones in the band. Erbium (in the form of an oxide) ispreferably selected as the main dopant and germanium, aluminium, andlanthanum (in the form of the respective oxides) are selected assecondary dopants.

The present invention also relates to an optical telecommunicationmethod comprising the steps of generating at least one optical signal ata predetermined wavelength in a wavelength band, supplying the signal toan optical fiber of an optical telecommunication line, amplifying theoptical signal at least once by at least one active-fiber opticalamplifier, and receiving the signal through a receiver. The active fiberof at least one of the amplifiers comprises a main fluorescent dopantand at least one secondary dopant interacting with the main dopant inthe glass matrix of the active fiber to generate an amplification gainof the optical signal at the predetermined wavelength in theactive-fiber optical amplifier, measured at an input power ≦-20 dBm,which differs by less than 1.6 dB from the corresponding gain of asignal at a different wavelength in the band, in the absence of filtermeans.

According to a third aspect, the method of the invention ischaracterized in that the optical signal/noise ratio at the receiver,measured at a 0.5 nm filter width is not lower than 15 dB, both for asingle signal in the band, and in the presence of two or more signals atdifferent wavelengths included in the band, simultaneously supplied tothe amplifier, for each of the optical signals. In particular, themethod includes the step of amplifying the optical signal at least twiceby means of respective active-fiber optical amplifiers interposed inseries along the optical fiber line.

In the telecommunication methods of the present invention, theactive-fiber optical amplifier preferably comprises an active fiberhaving a core doped with erbium as the main fluorescent dopant, the corebeing further doped with at least two secondary dopants interacting withthe main dopant and more preferably consisting of aluminium, germanium,and lanthanum, in the form of oxides.

The present invention in a fourth aspect relates to a telecommunicationsystem comprising a transmission station generating optical signals in apredetermined wavelength band, a receiving station, an optical fiberconnecting line between the transmission station and receiving station,and at least two active-fiber optical amplifiers connected in seriesalong the line. At least one of the optical amplifiers comprises asilica-based active optical fiber having a core doped with at least onemain fluorescent dopant and at least one secondary dopant, operativelyconnected with each other in such a manner that they supply thereceiving station with an optical signal/noise ratio, measured at a 0.5nm filter width, not lower than 15 dB for signals of a wavelengthincluded in the band, both for a single signal in the band and in thepresence of at least two signals of different wavelengths included inthe band and simultaneously supplied to the amplifier.

Preferably, the main fluorescent dopant of the glass core is erbium andthe secondary dopants are aluminium, germanium, and lanthanum.Conventionally, the erbium, aluminium, germanium, and lanthanum aredescribed to be present in the form of their respective oxides, asobtained by known manufacturing methods as described hereinafter. Thepredetermined transmission band is preferably included between 1530 and1560 nm, and the line according to the invention is preferably comprisedof at least three optical amplifiers connected in series with at leastone of the amplifiers having an active fiber the core of which is dopedwith aluminium, germanium, lanthanum and erbium, in their oxide forms.

Within the present invention, an active-fiber optical amplifier includesat least one length of silica-based active fiber, pumping means for theactive fiber, adapted to supply optical pumping power at a pumpingwavelength, and means for coupling within the active fiber the opticalpumping power and one or more transmission signals, at transmissionwavelengths included in a predetermined transmission band. The activeoptical fiber has a doped core with at least one main fluorescent dopantand at least one secondary dopant, in functional relation with eachother to such a degree that the maximum gain variation between twosignals at different transmission wavelengths in the band, measured atan input power ≦-20 dBm, is lower than 2.5 dB, in the absence offiltering means interposed along the active fiber.

In the amplifier, preferably the main fluorescent dopant is erbium, inthe form of an oxide, and preferably the secondary dopants arealuminium, germanium, and lanthanum, in the form of oxides. Inparticular, the active fiber optical amplifier has an emission curve, inthe predetermined wavelength band, which is substantially clear ofdepressions. Substantially clear of depressions means clear of portionsin the desired band of the emission spectrum having lower emission thanat adjacent wavelengths, where the amplification for a signal at thedepression wavelength is saturated at a lower wavelength in the presenceof signals at the adjacent wavelengths outside the depression. Thepredetermined transmission band is included between 1530 and 1560 nm andpreferentially between 1525 and 1560 nm. Preferentially the active fiberhas a numerical aperture greater than 0.15.

In another aspect of the present invention, an active optical fiber,suitable for optical telecommunications amplifiers, has a numericalaperture greater than 0.15 and a doped core with at least one mainfluorescent dopant and at least one secondary dopant, in functionalrelation with each other such that the emission curve of the fiber in apredetermined wavelength band, in the presence of optical pumping energysupplied to the fiber, is clear of depressions of a value higher than 1dB relative to the emission value in at least one of the adjacent zonesin the band. Preferably, no depression is higher than 0.5 dB. In theactive optical fiber, preferably the main fluorescent dopant is erbiumin the form of an oxide and preferably the secondary dopants arealuminium, germanium, and lanthanum in the form of oxides. In apreferred embodiment, the lanthanum content in the fiber core expressedas an oxide, is higher than 0.1% by mole and, more preferably, equal toor greater than 0.2% by mole. The germanium content in the fiber coreexpressed as an oxide is preferably higher than 5% by mole and the molarratio between the germanium content and lanthanum content in the fibercore, expressed as oxides, is included between 10 and 100 and, morepreferably, is about 50. The aluminium content in the fiber core,expressed as an oxide, is preferably higher than 1% by mole and, morepreferably, higher than 2% by mole. The erbium content in the fiber coreexpressed as an oxide is preferably between 20 and 5000 ppm by mole andmore preferably between 100 and 1000 ppm by mole. Preferably, thenumerical aperture of the fiber is higher than 0.18.

As shown in FIG. 1, an amplifier provided for use as a line amplifier,comprises one erbium-doped active fiber 1 and a respective pump laser 2,connected thereto by a dichroic coupler 3. An optical isolator 4 isdisposed upstream of the fiber 1, in the path direction of a signal tobe amplified, and a second optical isolator 5 is disposed downstream ofthe active fiber. Conveniently, although not necessarily, the dichroiccoupler 3 is located (as shown) downstream of the active fiber 1, sothat pump energy is supplied countercurrent to the signal.

The amplifier further comprises a second erbium-doped active fiber 6coupled with the output of a pump laser 7 by means of a dichroic coupler8, which may also provide countercurrent pumping in the example shown. Athird optical isolator 9 is present downstream of the fiber 6.

The pump lasers 2, 7 are preferably of the Quantum Well type and have anemission wavelength of λ_(p) =980 nm, and a maximum optical power at theexit of P_(u) =80 mW. Lasers of the above type are produced by LASERTRONINC., 37 North Avenue, Burlington, Mass. (U.S.).

The dichroic couplers 3, 8 are melted-fiber couplers formed ofsingle-mode fibers at 980 nm and made in the 1530-1560 nm wavelengthband, with variations <0.2 dB in the output optical power depending onpolarization. Dichroic couplers of the above type are known and may beof a type produced by GOULD Inc., Fiber Optic Division, Baymeadow Drive,Glen Burnie, Md. (U.S.) and SIFAM Ltd., Fiber Optic Division, WoodlandRoad, Torquay, Devon (GB).

The optical isolators 4, 5, 9 are optical isolators in which thepolarization control is independent of the transmission signalpolarization and have an isolation greater than 35 dB and a reflectancelower than -50 dB. A suitable isolator is a MDL I-15 PIPT-A S/N 1016model available from ISOWAVE, 64 Harding Avenue, Dover, N.J., U.S.

FIG. 2 shows an alternative embodiment of an amplifier in whichcorresponding elements are referred to by the same reference numerals asin FIG. 1. In such an amplifier, the components of which have the samefeatures as above described, a notch filter 10 is present, which isformed from an optical fiber portion having two optically-coupled coresand at a pre-selected wavelength. One of the cores is coaxial with theconnected optical fibers and the core is off-center and cut off at theends, as described in patents EP 441,211 and EP 417,441 the descriptionof which is herein incorporated by reference. This filter is sized sothat it couples in the off-center core a wavelength (or a wavelengthband) corresponding to a portion of the amplifier emission spectrumcutting off the off-center core at its ends enables the wavelengthtransferred thereto to be dispersed in the fiber cladding so that it isno longer re-coupled in the main core.

In the example shown, the two-core filter 10 has a wavelength bandcoupled in the second core BW(-3 dB) of 8-10 nm and a filter length of35 mm. The filter is sized to have maximum attenuation at the emissionpeak of the active fiber used. Alternatively, filters may be used whichhave attenuation at λ_(s) 1530 nm of 5 dB or attenuation at λ_(s) 1532nm of 11 dB. Such a filter reduces the intensity of a specificwavelength zone, in particular the fiber emission peak, in order toobtain a gain curve of the amplifier that is as constant (or "flat") aspossible over varying wavelengths. This requirement is particularlyimportant in WDM telecommunications in which amplification conditionsthat are as uniform as possible are desired for each channel.

For use in the above described amplifiers, different types oferbium-doped active fibers were used and, the compositions and opticalfeatures of the fibers are summarized in the following table

                                      TABLE 1                                     __________________________________________________________________________    Al.sub.2 O.sub.3                                                                        GeO.sub.2                                                                             La.sub.2 O.sub.3                                                                     Er.sub.2 O.sub.3                                                                        λ.sub.c                             Fiber                                                                            wt %                                                                             (mol %)                                                                           wt %                                                                              (mol %)                                                                           wt %                                                                             (mol %)                                                                           wt %                                                                             (mol %)                                                                           NA nm                                         __________________________________________________________________________    A  4  (2.6)                                                                             18  (11.4)                                                                            1  (0.2)                                                                             0.2                                                                              (0.03)                                                                            0.219                                                                            911                                        B  1.65                                                                             (1.1)                                                                             22.5                                                                              (14.3)                                                                            0  (0) 0.2                                                                              (0.03)                                                                            0.19                                                                             900                                        C  4  (2.6)                                                                             18  (11.4)                                                                            0  (0) 0.2                                                                              (0.03)                                                                            0.20                                                                             1025                                       D  4  (2.6)                                                                             0   (0) 3.5                                                                              (0.7)                                                                             0.2                                                                              (0.03)                                                                            0.19                                                                             900                                        __________________________________________________________________________

wherein:

wt %=(average) percent content by weight of oxide in the core

mol %=(average) percent content by mole of oxide in the core

NA=Numerical Aperture (n1² -n2²)^(1/2)

λ_(c) =cut-off wavelength (LP11 cut-off).

Analyses of the compositions were made on a preform (before fiberdrawing) by a microprobe combined with a Hitachi scanning electronmicroscope (SEM). Analyses were conducted at magnifications of 1300 ondiscrete points disposed along a diameter and separated from each otherby 200 μm. The fibers were made by the vacuum plating method, within aquartz glass tube. In the fibers, the incorporation of germanium as thedopant into the SiO₂ matrix in the fiber core is obtained during thesynthesis step.

The incorporation of erbium, alumina, and lanthanum into the fiber corewas obtained by the "solution doping" technique, in which an aqueoussolution of the dopant chlorides is put into contact with the synthesismaterial of the fiber core, while it is in a particulate state (alsocalled soot), before consolidation of the preform.

In particular, one or more cladding layers are first deposited on theinside surface of the substrate tube in a lathe, following which silicacore layers are deposited at a reduced temperature to form a partiallysintered porous soot. The alumina and erbium (and lanthanum in thiscase) dopants are then introduced by removing the thus formed tube fromthe lathe and soaking the core layers in an aqueous or alcoholicsolution of aluminium, erbium, and lanthanum salts to ensure saturationof the porous soot. The tube is then replaced in the lathe, the corelayers are dried and fused, and the tube collapsed. Fibre drawing fromthe thus formed preform is performed in a conventional manner.

More details on the solution doping technique can be found for examplein U.S. Pat. No. 5,282,079.

The greater numerical aperture value (NA) of fiber A relative to thecomparison fibers was caused by the fact that, in making the fiber core,the modification of the reagent flow previously adopted for making fiberC (Al/Ge/Er) was omitted, in particular, the germanium supply was notclosed. The subsequent incorporation of lanthanum and aluminium bysolution doping brought the value of the refractive index of the corehigher than expected, in addition to the unexpected advantages achievedin terms of amplification and transmission to be described.

The experimental configuration adopted for determining the spectralemission of the fibers is diagrammatically shown in FIG. 3, and thespectral emission graphs measured on the active fibers A, B, C, D arereproduced in FIG. 4.

A pump laser diode 11, at 980 nm, was connected through a 980/1550dichroic coupler 12 to the active test fiber 13, and the fiber emissionwas detected through an optical spectrum analyzer 14. The laser diode 11had a power of about 60 mW (in the fiber 13). The active fiber 13 had alength corresponding to most efficient amplification for the adoptedpump power. All the fibers had the same erbium content with lengths ofabout 11 m. For different erbium contents in the fibers, an appropriatelength can be determined by adopting criteria known to a person skilledin the art. The optical spectrum analyzer was a TQ8345 model produced byADVANTEST CORPORATION, Shinjuku--NS Bldg., 2-4-1 Nishi-Shinjuku,Shinjuku-ku, Tokyo (JP).

Measurements were carried out by pumping the fiber at 980 nm anddetecting the spontaneous emission spectrum of the fiber. The resultsare shown in FIG. 4 wherein curve 15 corresponds to fiber A, curve 16corresponds to fiber B, curve 17 corresponds to fiber C, and curve 18corresponds to fiber D. As noted from the graphs, the spectrum emissionsfrom fibers B, C, D have main peaks of great intensity with a maximum atabout 1532.5 nm and a subsequent zone of high emission at higherwavelengths, until about 1560-1565 nm, inclusive of a secondary greatlybroadened peak.

Comparing curves 16 and 17 (fibers B and C, respectively), it can beseen that a greater alumina content in the fiber raises the level of thehigh-emission zone, and the substitution of lanthanum for germanium(fiber D, curve 18) enables a still higher level to be reached in the1535-1560 nm range. On the other hand, in each of fibers B, C, and D,the presence of a depression was observed in a zone d in the spectrum(localized between about 1535 and 1540 nm), included between the mainemission peak and contiguous thereto, and the secondary emission peak.The depression indicates an emission value lower by at least 2 dB thanthe maximum emission value in the adjacent zones (that is both the mainpeak and the secondary peak), as shown in the figure by reference h forcurve 16 alone, but clearly identifiable for curves 17 and 18 too.

On the contrary, curve 15 shows that under the experimental conditionsshown, fiber A in zone d does not show a significant depression in thespectrum (or, where a depression is detectable, it is at all eventslower than about 0.5 dB). Curve 15 also shows that the maximum-emissionpeak of fiber A is at lower wavelengths than fibers B, C, and D, beinglocalized at about 1530 nm, and that the fiber holds a high emissionlevel as far as close to 1520 nm.

Amplifiers of the structure shown in FIGS. 1 and 2 were made using fiberA. The first active fiber 1 was about 8 m long, whereas the secondactive fiber 6 was about 15 and 13 m long in the cases of FIG. 1 andFIG. 2, respectively. FIG. 5 shows the gain curves at differentwavelengths, for two different input power levels, for the amplifier ofFIG. 1, and FIG. 6 shows the gain curves at those different wavelengthsfor the amplifier shown in FIG. 2, for three different input powerlevels.

In particular, curve 19 in FIG. 5 refers to a -20 dBm input power,whereas curve 20 refers to a -25 dBm input power in the amplifier ofFIG. 1. Curve 21 in FIG. 6, refers to a -20 dBm input signal power, inthe amplifier of FIG. 2, curve 22 refers to a -25 dBm input signalpower, and curve 23 refers to a -30 dBm input signal power.

As can be seen from the figures, in particular by comparing curves 19and 21, corresponding to a -20 dBm power level which is of particularinterest for telecommunications, both in the absence and in the presenceof a filter, the use of a fiber having a core doped with alumina,germanium and lanthanum, in addition to erbium, enables a substantiallyflat gain curve to be achieved, particularly at the zone between 1536and 1540 nm, which result can also be reached in the absence of afilter.

In particular, in the absence of a filter at -20 dBm, the gaindifference between signals at different wavelengths was lower than 1.6dB, whereas in the presence of a filter, at -20 dBm, the gain differencebetween signals at different wavelengths was lower than 0.9 dB.

FIG. 7 shows the gain curves at different wavelengths for threedifferent input power levels of an amplifier having the structure shownin FIG. 2, made from fiber C (Al/Ge/Er). In particular, curve 24 of FIG.7 refers to a -20 data input signal power, curve 25 refers to a -25 datainput signal power, and curve 26 refers to a -30 dBm input signal power.At -20 data the gain difference between signals at different wavelengthswas about 2.1 dB. As discernible by comparison, fiber A (Al/Ge/La/Er)also in an amplifier devoid of filter, gives rise to a much flatter gaincurve than fiber C (Al/Ge/Er) in an amplifier provided with a filter.

Using the amplifiers of FIG. 1 and FIG. 2, made either of fiber A(Al/Ge/La/Er) or of fiber C (Al/Ge/Er), transmission tests over longdistances with several amplifiers in cascade, that is seriallyconnected, were carried out. One experimental configuration used isshown in FIG. 8. A first signal 27, at a wavelength λ₁ =1536 nm, and asecond signal 28, at a wavelength λ₂ =1556 nm were fed to a fiber 29through a multiplexer 30. One attenuator 31 was present downstream of apower amplifier 32a. Other subsequent attenuators 31, equal to eachother, were disposed on the line along which four amplifiers 32, 32',32", 32"' in succession were disposed before a receiver 33. The receiver33 was preceded by an optical demultiplexer 34 having an interferentialfilter with a band width of 1 nm at -3 dB, by which the detectedwavelength was selected. Signals 27, 28, generated by respective lasers,each had 0 dBm power, and the overall power multiplexed in fiber 29 wasof 0 dBm (as a result of a 3 dB coupling loss).

Multiplexer 30 was a "coupler 1×2" produced by E-TEK DYNAMMICS INC.,1885 Lundy Avenue, San Jose, Calif. (U.S.). The power amplifier 32a wasa fiber optic amplifier commercially available and having the followingfeatures: input power from -5 to +2 dBm; output power 13 dBm; and workwavelength 1530-1560 nm.

The power amplifier 32a was devoid of a notch filter, amplifier model.TPA/E-12 model available from the assignee of the present applicationwas used. The amplifier utilized a C type (Al/Ge/Er) erbium-doped activeoptical fiber, and by power amplifier is intended to mean an amplifieroperating under saturation conditions, in which the output power dependson the pumping power, as described in detail in patent EP 439,867incorporated herein by reference. After the first attenuator 31, at theamplifier 32 input, the overall optical power was about -18 dBm.

As the attenuators 31, a model Va5 available from JDS FITEL INC., 570Heston Drive, Nepean (Ottawa), Ontario (CA) was used and each of themsupplied a 30 dB attenuation to emulate about 100 km of optical fiber.Amplifiers 32, 32', 32", 32"' were identical and each of them supplied again of about 30 dB for both wavelengths λ₁ and λ₂, at an overall outputpower of +12 dBm.

Signal 27, at wavelength λ₁ =1536 nm, was a signal directly modulated at2.5 Gbit/s, generated by a DFB laser, incorporated in the SLX-1/16 ModelSDH terminal apparatus, commercially available from PHILIPS NEDERLANDBV, 2500BV Gravenhage (NL). Signal 28, at wavelength λ₂ =1556 nm, was acontinuous signal (CW), generated by a MG0948L3 model DFB laser, of 0dBm power, produced by ANRITSU CORPORATION, 5-10-27 Minato-ku, Tokyo(JP). The interferential filter 34 was a TB4500 model, produced by JDSFITEL INC.

Experiment 1

In a first experiment amplifiers with fiber A (Al/Ge/La/Er) having theconfiguration shown in FIG. 1, i.e., devoid of the notch filter 10, wereused.

Experiment 2

In a second experiment amplifiers with fiber A (Al/Ge/La/Er) having theconfiguration shown in FIG. 2, i.e., including a notch filter 10, wereused. Through the receiver 33 the bit error rate (BER) was measured byvarying of the average reception power, for the signal at the λ₁ (1536nm) wavelength. The results are shown in the diagram in FIG. 9, in whichcurve 35 relates to experiment 1 and curve 36 relates to experiment 2.

As shown in FIG. 9, in spite of the fact that the gain curve of a singleamplifier with fiber A (Al/Ge/La/Er) provided with a notch filter wassubstantially identical with, and even flatter than that of a singleamplifier with fiber A without a notch filter 10, in a cascadeconfiguration, the signal at 1536 nm appeared to be penalized because itincluded a remarkably higher error rate at the same levels of receptionpower.

Experiment 3

A third experimental configuration was used as shown in FIG. 10. In thistest, four signals 37, 38, 39, 40 at wavelengths λ₁ =1536 nm, λ₂ =1556nm, λ₃ =1550 nm and λ₄ =1544 nm, respectively, were fed to a fiber 41through a wavelength multiplexer 42. The signal level at the line entrywas adjusted through a pre-equalizer 43. After passing through a poweramplifier 44 the signals were sent to a line including four lineamplifiers 45, 45', 45", 45"' having respective attenuators 46interposed therebetween to simulate lengths of optical fibers. Thereceiving station was comprised of a preamplifier 47, an opticaldemultiplexer 48 and a receiver 49.

The signals 37, 38, 39 and 40 were respectively generated from a DFBlaser at 1536 nm, directly modulated at 2.5 Gbit/s, incorporated in theterminal apparatus constituting the receiver 49; from a DFB laser at1556 nm, of the continuous-emission type, produced by ANRITSU; from aDFB laser at 1550 nm, of the continuous-emission type, produced byANRITSU; and from an ECL laser, at a variable wavelength preselected to1544 nm, of the continuous-emission type, model HP81678A, produced byHEWLETT PACKARD COMPANY, Rockville, Md. (U.S.).

The pre-equalizer 43 included four variable attenuators 43a, produced byJDS, the attenuation of which was set depending on the optical power ofthe respective channel. The multiplexer 42 included a 1×4 splitterproduced by E-TEK DYNAMICS.

The power amplifier 44 was the already described model TPA/E-12. Theamplifiers 45, 45', 45", 45"' were identical with each other and eachprovided a gain of about 30 dB, at an overall output power of +12 dBm.The amplifiers 45 had the structure shown in FIG. 1 and utilized fiber A(Al/Ge/La/Er). The attenuators 46 each provided an attenuation of 30 dB,corresponding to about 100 km of optical fiber and were produced by theaforementioned JDS FITEL.

The preamplifier 47 was a commercially available optical preamplifierhaving the following features: gain 22 dB; noise factor <4.5 dB; outputpower from -26 to -11 dB; and work wavelength 1530-1560 nm. ModelRPA/E-F, commercially available from the assignee of this applicationwas used, and the amplifier utilized an active fiber type C (Al/Ge/Er).Preamplifier is intended to mean an amplifier sized for receiving asignal having a very low intensity (-50 dBm, for example) and amplifyingit, before sending it to a receiving device, until a power level adaptedto the device responsiveness is achieved.

The optical demultiplexer 48 included a wavelength-tunable Fabry-Perotfilter, having a band width of 0.8 nm at -3 dB, incorporated in thepreamplifier 47. For carrying out the experiment, the Fabry-Perot filterwas tuned to a wavelength λ=1536 nm (identified as a criticalwavelength) through a pilot tone generated from the transmitter 37. Thereceiver 49 was an end SDH apparatus, model SLX-1/16, commerciallyavailable from PHILIPS NEDERLAND BV, 2500BV Gravenhage (NL).

FIGS. 11 to 15 show the signal course in the subsequent stages, inparticular at the input of amplifier 45, amplifier 45', amplifier 45"and amplifier 45"' respectively and at the input of the preamplifier 47.The pre-equalizer 43 applied a maximum starting pre-equalization ofabout 7 dB between the different channels, as shown in FIG. 11, tocompensate for the saturation effects at the lower wavelengths takingplace in cascade amplifiers. The pre-equalization was carried out insuch a manner that the optical signal/noise (S/N) ratios at thepreamplifier 47 exit could be equalized. In the subsequent amplificationstages, one can see a reduction in the gain curve in the region having asmaller wavelength, due to the above described saturation phenomenon,whereas the optical S/N ratio of each of the channels remained high(S/N≧15 dB with Δλ=0.5 nm) until the preamplifier 47 exit.

In a corresponding experiment carried out using amplifiers according tothe diagram in FIG. 2 having an active fiber of the C type and a notchfilter, an important reduction in the signal power at 1536 nm and 1544nm was found, as well as a strong unbalance in the optical S/N ratiosbetween the different channels, as is apparent from the graph in FIG.16, showing the powers of the different channels at the preamplifierinput. A still larger reduction can be found for a channel at about 1540nm of wavelength.

In this case pre-equalization would have enabled the unbalance betweenthe different channels (some of which appeared greatly reduced inrespect of others and in particular those between about 1535 and 1540nm) to be restrained. However, by carrying out such equalization, anacceptable S/N ratio for all signals in the wavelength band of interestcould not be maintained at all events. In fact, to pre-equalize thechannels a very high starting attenuation of the most favoredchannels=(1550 and 1556 nm), would be necessary which would haveresulted in an S/N ratio of very low value (in the order of 8-10 dB),thereby making a correct reception of the signals themselves impossible.

The better results achieved with fiber type A as compared to the use ofamplifiers provided with a notch filter and an Al/Ge/Er fiber are deemedto be due to the fact that fiber A has an emission curve practicallyclear of depressions or local a minimum of an important amount and inparticular devoid of a minimum in the wavelength range contiguous to theemission peak, in the 1535-1540 nm zone. When several signals atdifferent wavelengths are simultaneously fed to the fiber, the presenceof a depression or local minimum in the emission curve (apparent in thespectra of the comparative fibers) causes a signal of a wavelengthcorresponding to the depression to be amplified to a smaller extent thanthe signals at the wavelengths of the adjacent ranges.

According to the above interpretation, the greater signal amplificationat the wavelengths of adjacent ranges subtracts pumping energy to thesignal itself which is saturated to a low output value (that is itslevel after amplification no longer depends on its input value, but onlyon the pump power available in the fiber), to increment the leveldifference between the different signals. In the presence of cascadeamplifiers and in WDM transmissions, such a phenomenon is incremented ateach stage and it is deemed to be responsive to the detected unevennessin the response, which cannot be compensated for by pre-equalization orthe like.

It has been noted that the above phenomenon takes place for signals atthe depression of the emission curve, due to the signal gain competitionat wavelengths adjacent to the depression wavelength. This does notoccur (at least to the same extent) for signals at wavelengths locatedat the limits of the useful band, although at such wavelengths theemission value may be absolutely equal to or lower than the value of thedepression.

According to the present invention, the incorporation of lanthanum intoan Al/Ge/Er fiber has unexpectedly eliminated such local emissionminimum. This could not have been foreseen based on the available datafor Al/La/Er and Al/Ge/Er fibers. In fact, both Al/La/Er and Al/Ge/Erfibers have an important emission depression in the 1535-1540 nm zoneand knowing the performance of such known fibers would not haveindicated a different, favorable behavior for an Al/Ge/La/Er fiber andin addition that such a fiber would have enabled amplified wavelengthmultiplexing transmission.

Unexpectedly, according to another and still more important aspect, ithas been found that in the presence of a peak within a high-emissionzone, the presence of the depression contiguous to the peak or at allevents in functional (negative) relation with the adjacent zones, wasresponsible for an insufficient value in the signal/noise ratio forsignals in the depression and that an active fiber capable ofintrinsically eliminating or reducing the depression could solve theproblem by enabling WDM transmission in the presence of one or moreamplifiers. Therefore, according to the present invention, it has beenfound that an active fiber doped to give an emission curve having arelatively high value in a wavelength band, substantially clear of localdepressions in a zone within the wavelength band in functional relationwith the remaining zones of the band (that would generate an importantgain difference for telecommunications signals at different wavelengthswithin the band of wavelengths multiplexed in the fiber itself) resultedin amplifiers particularly adapted to be used to provide highperformance in a telecommunications line comprising at least twoserial-connected optical amplifiers, with wavelength divisionmultiplexed signals.

In another aspect, according to the present invention, it has been foundthat the S/N ratio control in transmission systems as referred to above,can be obtained not only with the use of filters or by adopting atransmission band of restricted width (capable of avoiding includingdisfavored wavelength zones), but through the dopant choice and dosingin the active fiber core of the amplifier, such that an emission curvemay be drawn in a band wide enough (that is extended as far as 1525-1560nm, or at least 1530-1560 nm) not to give rise to an disfavored signalamplification in one or more particular zones in the emission curve,although an emission peak is present in the band. Functional relationmeans, as above explained, that the presence of a greater emission inthe zones adjacent to the depression, and in particular of an emissionpeak, and the presence of signals in the adjacent zones causes areduction in the amplification of a signal at a wavelength correspondingto the depression.

An emission (or spectrum) curve has a relatively high value in awavelength band when a given wavelength band, preferably between 1525and 1560 nm, shows an emission of a value exceeding the emission outsidethe band to enable the amplification of a signal in the wavelength band.Such a zone is identified as the zone included between two end values,at which the emission is 3 dB lower than that at a wavelength includedin the interval or band (preferably in a practically constant zone ofthe interval). In actual fact, such a band corresponds to that at whichuseful amplification can be carried out.

Emission peak is intended to mean an emission in a wavelength rangewhich is greatly higher than in the spectrum zones outside such a range,so that different behaviors occur with respect to signals fed to thefiber at wavelengths within and outside this range. Important gaindifference is intended to mean, for example, a difference higher than 2dB between the most favored wavelength and the less favored wavelengthin the band (at an input power equal to or lower than -20 dBm).

Local depressions of the emission curve is intended to mean a wavelengthrange within the band at which there is a secondary emission minimum, ofa lower value than the emission value at either limit of the range andof a lower value by a predetermined amount than the maximum emissionvalues in the contiguous wavelength ranges (in particular both the mainemission peak of erbium, at wavelengths lower than the depression ones,and the secondary emission peak at higher wavelengths). For the purposeof the present invention, values of the predetermined depression amountwhich are higher than 0.5 dB and, more particularly, higher than 1 dBgive noticeable effects.

It has been also found that, in a line amplifier used in a systemprovided with several cascade amplifiers, the use of a notch filter,while capable of restricting the intensity of the main emission peak bygenerating a substantially flat gain curve for the individualamplifiers, does not enable the above described phenomenon to beavoided. In fact, a notch filter, in a configuration involving severalcascade amplifiers is deemed to constitute an attenuating element in theband zone at low wavelengths where it is centered. The effect inevitablyextends to the depression zone of the emission curve. The attenuationeffect arises in addition to the above described saturation phenomenonand generates a further penalization for a signal at a wavelength insuch a depression or local minimum. The use of equivalent filter meansadapted to attenuate or otherwise restrain the emission at the mainpeak, such as described in the above mentioned EP 426,222 for example,is deemed not to lead to important differences in performance.

For purposes of the present invention, the lanthanum content in thefiber core is preferentially higher than 0.1% by mole and the germaniumcontent is higher than 5% by mole. The Ge/La ratio is preferablymaintained at 50 and at all events included between 10 and 100. Thepresence of lanthanum in the fiber core enables a greater incorporationof germanium and alumina into the fiber, so that a high numericalaperture (higher than 0.18 and preferably at least equal to 0.2) isachieved, which brings about important advantages in terms ofamplification efficiency and a more constant response in the band. Thepresence of lanthanum, in addition, enables the erbium content in thefiber to be increased without giving rise to clustering phenomena. Theerbium content may be included between 20 and 5000 ppm, or morepreferably between 100 and 1000 ppm.

While described in detail with reference to the use in line amplifiers,the fibers in accordance with the present invention can convenientlyalso be employed in a preamplifier. Such an amplifier is preferablysized for receiving a signal of very low intensity (-50 dBm for example)and amplifying it before it is sent to a receiving device. In additionit is noted that, while optical two-stage amplifiers have been describedwhich use two successive and separately pumped portions of active fiber,in accordance with the present invention single-stage amplifiers mayalso be made, for example following the construction diagrams shown inthe aforementioned EP 426,222 and EP 439,867 patents and amplifiersdifferent in type from each other, for example mono-stage and two-stageamplifiers, can be used together in one and the same connection.

In addition, for specific requirements, in two-stage amplifiers it ispossible that only one of them be made with the fiber of the presentinvention.

On the other hand, a person skilled in the art, taking into account theabove considerations, will be able to identify specific operatingconditions and specific dopant contents adapted to the intendedapplication for the purpose of achieving the states response results.

Within the present invention, a person skilled in the art operating withfibers containing a main dopant (preferably erbium when thetelecommunications field is concerned) which is fluorescent in thewavelength range of interest, in combination with secondary dopantsinteracting therewith in an additive or interoperative manner, will beable to identify specific dopants or combinations thereof and relevantdosages, in order to obtain variations in the emission curve of thefiber, and corresponding performances of amplifiers and amplifiedsystems made thereby (lasers, optical gyroscopes, and the like, as wellas transmission, telecommunications or measurement systems embodyingthem) in order to obtain the desired performance in terms ofsignal/noise ratio within the band of interest. In the specific fieldwhich is particularly of interest for the Applicant, the research hasbeen limited to erbium as the main fluorescent dopant, and to Ge, Al,and La incorporated into the fiber in the form of oxides, as thesecondary dopants, because the results of this research have beensufficient to solve the specific technical problems.

The teachings given in the present invention will be used by a person ofordinary skill in the art in order to solve problems which may besimilar to, or different from, those herein described, provided thatthey have the same technical grounds, through the research of specificdifferent dopants or particular dosages as herein experimented anddescribed, putting into practice or using the same functional relationbetween results and employed means.

What is claimed is:
 1. An active optical fiber for use in a laser-pumpedoptical telecommunications amplifier comprising:a core; at least onemain fluorescent dopant in said core; and at least one secondary dopantin said core, said main dopant said secondary dopant having relativecharacteristics such that emission power of the fiber in a predeterminedwavelength band includes a main emission peak and a secondary emissionpeak when optical pumping energy is supplied to the fiber, the emissionpower at the secondary emission peak not exceeding by more than 1 dB theemission power at all wavelengths between the main emission peak and thesecondary emission peak, wherein said main fluorescent dopant is erbiumin the form of an oxide, and wherein said secondary dopant includes anoxide of aluminium, an oxide of germanium, and an oxide of lanthanum. 2.An active optical fiber according to claim 1, wherein the lanthanumcontent in the fiber core expressed as an oxide is higher than 0.1% bymole.
 3. An active optical fiber according to claim 1, wherein thelanthanum content in the fiber core expressed as an oxide is equal to orgreater than 0.2% by mole.
 4. An active optical fiber according to claim1, wherein the germanium content in the fiber core expressed as an oxideis higher than 5% by mole.
 5. An active optical fiber according to claim4, wherein the molar ratio between the lanthanum content and germaniumcontent in the fiber core expressed as oxides, is between 10 and
 100. 6.An active optical fiber according to claim 4, wherein the molar ratiobetween the lanthanum content and germanium content in the fiber coreexpressed as oxides is about
 50. 7. An active optical fiber according toclaim 1, wherein the aluminium content in the fiber core, expressed asan oxide is higher than 1% by mole.
 8. An active optical fiber accordingto claim 1, wherein the aluminium content in the fiber core expressed asan oxide is higher than 2% by mole.
 9. An active optical fiber for usein a laser-pumped optical telecommunications amplifier comprising:acore; at least one main fluorescent dopant in said core; and at leastone secondary dopant in said core, said main dopant and said secondarydopant having relative characteristics such that emission power of thefiber in a predetermined wavelength band includes a main emission peakand a secondary emission peak when optical pumping energy is supplied tothe fiber, the emission power at the secondary emission peak notexceeding by more than 1 dB the emission power at all wavelengthsbetween the main emission peak and the secondary emission peak, andwherein the erbium content in the fiber core expressed as an oxide isbetween 20 and 5000 ppm by mole.
 10. An active optical fiber for use ina laser-pumped optical telecommunications amplifier comprising:a core;at least one main fluorescent dopant in said core; and at least onesecondary dopant in said core, said main dopant and said secondarydopant having relative characteristics such that emission power of thefiber in a predetermined wavelength band includes a main emission peakand a secondary emission peak when optical pumping energy is supplied tothe fiber, the emission power at the secondary emission peak notexceeding by more than 1 dB the emission power at all wavelengthsbetween the main emission peak and the secondary emission peak, andwherein the numerical aperture of the fiber is higher than 0.15.
 11. Anactive optical fiber comprising a core doped with a main fluorescentdopant and a secondary dopant, said main fluorescent dopant comprisingerbium oxide and the secondary dopant comprising an oxide of aluminium,an oxide of germanium and an oxide of lanthanum.
 12. An active opticalfiber according to claim 11, wherein the lanthanum content of the fibercore is higher than 0.1% by mole.
 13. An active optical fiber accordingto claim 12, wherein the germanium oxide content of the fiber core ishigher than 5% by mole.
 14. An active optical fiber according to claim13, wherein the aluminum oxide content of the fiber core is higher than1% by mole.
 15. An active-fiber optical amplifier, comprising:at leastone length of silica-based active fiber; a pump for supplying opticalpumping power at a pumping wavelength to said active fiber; a couplercoupling said optical pumping power and at least one transmission signalat a transmission wavelength to said active fiber, a predeterminedtransmission band including said transmission wavelength; said activeoptical fiber having a core doped with at least one main fluorescentdopant and at least one secondary dopant, said main fluorescent dopantand said secondary dopant having relative characteristics such that amaximum gain variation between any and all two different transmissionwavelengths in said predetermined transmission band measured at an inputpower less than or equal to -20 dBm, is lower than 2.5 dB, wherein thepredetermined transmission band includes 1535 nm to 1545 nm.
 16. Anactive-fiber optical amplifier according to claim 15, wherein said mainfluorescent dopant is an oxide of erbium.
 17. An active-fiber opticalamplifier comprising:at least one length of silica-based active fiber; apump for supplying optical pumping power at a pumping wavelength to saidactive fiber; a coupler within said active fiber to couple said opticalpumping power and at least one transmission signal, at a transmissionwavelength included in a predetermined transmission band, to said activefiber; said active optical fiber having a core doped with at least onemain fluorescent dopant and at least one secondary dopant, said mainfluorescent dopant and said secondary dopant having relativecharacteristics such that a maximum gain variation between two signalsat different transmission wavelengths in said predetermined transmissionband measured at an input power less than or equal to -20 dBm, is lowerthan 2.5 dB, wherein said main fluorescent dopant is an oxide of erbium,and wherein said secondary dopant comprises oxides of aluminium,germanium, and lanthanum.
 18. An active-fiber optical amplifieraccording to claim 15, wherein emission power of the fiber in saidpredetermined transmission band includes a main emission peak and asecondary emission peak, the emission power at the secondary emissionpeak not exceeding by more than 1 dB the emission power at allwavelengths between the main emission peak and the secondary emissionpeak.
 19. An active-fiber optical amplifier according to claim 18,wherein said emission curve has depressions of a value not higher than0.5 dB with respect to the emission value in at least one of theadjacent zones in said band.
 20. An active-fiber optical amplifieraccording to claim 15, wherein said predetermined transmission band isbetween 1525 and 1560 nm.
 21. An active-fiber optical amplifieraccording to claim 15, wherein said active fiber has a numericalaperture greater than 0.15.
 22. An active fiber optical amplifier,comprising:a first silica-based active-fiber; a first pump for supplyingoptical pumping power at a first pumping wavelength to said first activefiber; a first coupler for coupling the optical pumping power of saidfirst pump and a first transmission signal at a first transmissionwavelength included in a predetermined transmission band to said firstactive fiber; a second silica-based active fiber; a second pump forsupplying optical pumping power at a second pumping wavelength to saidsecond active fiber; a second coupler for coupling the optical pumpingpower of said second pump and a second transmission signal at a secondwavelength included in said predetermined transmission band to saidsecond active fiber; wherein at least one of said first active fiber andsaid second active fiber includes a core doped with a main fluorescentdopant and at least one secondary dopant, said main and secondarydopants having relative characteristics such that a maximum gainvariation between any and all two different transmission wavelengths forsaid first transmission signal and said second transmission signalmeasured at an input power less than or equal to -20 dBm is lower than2.5 dB, wherein the predetermined transmission band includes 1535 nm to1545 nm.
 23. An active fiber optical amplifier comprising:at least onelength of silica-based active fiber; a pump for supplying opticalpumping power at a pumping wavelength to said active fiber; a couplerwithin said active fiber to couple said optical pumping power and atleast one transmission signal, at a transmission wavelength included ina predetermined transmission band, to said active fiber; said activeoptical fiber having a core doped with at least one main fluorescentdopant and at least one secondary dopant, said main fluorescent dopantand said secondary dopant having relative characteristics such that amaximum gain variation between two signals at different transmissionwavelengths in said predetermined transmission band measured at an inputpower less than or equal to -20 dBm, is lower than 2.5 dB, wherein saidpredetermined transmission band is between 1525 and 1560 nm, and whereinsaid main fluorescent dopant is an oxide of erbium and said secondarydopant includes an oxide of aluminium, an oxide of germanium, and anoxide of lanthanum.
 24. An active-fiber optical amplifier according toclaim 19, characterized in that said active fiber has a numericalaperture greater than 0.15.
 25. An active-fiber optical amplifier,comprising:at least one length of silica-based active fiber; a pump forsaid active fiber to provide optical pumping power; a coupler withinsaid active fiber to couple said pumping power and at least onetransmission signal, at a transmission wavelength included in apredetermined transmission wavelength band, to said active fiber, saidactive fiber having a core doped with at least one main fluorescentdopant and at least one secondary dopant, said main fluorescent dopantand said secondary dopant having relative characteristics such that anemission curve of the active fiber in said predetermined transmissionwavelength band is substantially clear of a depression having a loweremission value than at adjacent wavelengths in said predeterminedtransmission wavelength band, where the amplification for a signal at adepression wavelength in said emission curve is saturated at a loweroutput value in the presence of signals at said adjacent wavelengthsoutside the depression.
 26. A telecommunications system for transmittingoptical signals in a predetermined wavelength band from a transmissionstation to a receiving station, comprising:an optical fiber adapted toconnect the transmission station and receiving station; and at least twoactive-fiber optical amplifiers connected in series along said opticalfiber; at least one of said optical amplifiers comprising a silica-basedactive optical fiber having a core doped with at least one mainfluorescent dopant and at least one secondary dopant in appropriateamounts such that the receiving station receives signals with an opticalsignal/noise ratio, measured at a 0.5 nm filter width, greater than orequal to 15 dB for signals of a wavelength included in saidpredetermined wavelength band, wherein the secondary dopant includes anoxide of lanthanum.
 27. A telecommunication system according to claim26, wherein at least two signals at different wavelengths are includedin said predetermined wavelength band and are simultaneously fed to saidat least one of said optical amplifiers.
 28. A telecommunication systemaccording to claim 26, wherein said main fluorescent dopant is an oxideof erbium.
 29. A telecommunication system according to claim 28, whereinsaid secondary dopant includes an oxide of aluminium and, an oxide ofgermanium.
 30. A telecommunication system according to claim 28, whereinthe predetermined transmission band is between 1530 and 1560 nm.
 31. Atelecommunication system according to claim 26, further including athird optical amplifier connected in series along the optical fiber. 32.A telecommunication system according to claim 31, wherein at least oneof said three serially connected amplifiers has an active fiber, thecore of said active fiber being doped with an oxide of aluminium, anoxide of germanium, an oxide of lanthanum, and an oxide of erbium. 33.An optical telecommunication method comprising the steps of:generatingat least one optical signal at a predetermined wavelength in awavelength band; supplying the optical signal to a length of an opticalfiber for transmission therein within an optical telecommunication line;providing in the length of the optical fiber an active-fiber opticalamplifier including an active fiber doped with a main fluorescent dopantand at least one secondary dopant interacting with the main dopant inthe active fiber to generate an amplification gain of the optical signalat the predetermined wavelength in the active-fiber optical amplifier,measured at an input power less than or equal to -20 dBm, differing byless than 1.6 dB from the corresponding gain of any and all signals at adifferent wavelength in the band, wherein the predetermined transmissionband includes 1535 nm to 1545 nm; amplifying the optical signal at leastonce in the length of the optical fiber by at least one active-fiberoptical amplifier; and receiving the at least once amplified opticalsignal through a receiver.
 34. A telecommunication method according toclaim 33, wherein at least two signals at different wavelengths areincluded in the predetermined wavelength band and are simultaneously fedto said optical amplifier.
 35. A telecommunication method according toclaim 33, further including amplifying the optical signal at least twiceby providing at least two active-fiber optical amplifiers in the lengthof the optical fiber.
 36. A telecommunication method according to claim33, wherein the wavelength band extends from 1530 to 1560 nm.
 37. Atelecommunication method according to claim 33, wherein the wavelengthband extends from 1525 to 1560 nm.
 38. A telecommunication methodaccording to claim 33, wherein erbium is the main fluorescent dopant.39. A telecommunication method according to 38, wherein the secondarydopants include an oxide of aluminium, an oxide of germanium, and anoxide of lanthanum.